A communication system may comprise several base stations each covering a cell area. The base stations, BS, may be connected to a Radio Network Controller, RNC. Several User Equipments, UE, may be located within the cell areas of the base stations.
In a Code Division Multiple Access CDMA system, communication takes place simultaneously for several UE:s over the same carrier frequency. Each UE has a separate channel defined by a user specific code over which communication takes place, according to methods well known in the art. Each UE contributes to the interference in said system. Thus, there is a desire to keep the power emitted from each UE as low as possible, while still maintaining satisfactory quality of the transmission.
One possibility to achieve such power control of the UE is by controlling the up-link transmit power from the UE:s so that a specific Bit Error Rate BER or Block Error Rate BLER of the received signal at the base station is obtained. This may be performed by determining a target Signal-to-Interference ratio SIR at the base station side corresponding to the required BER or BLER. An Outer Loop Power Control OLPC system may be arranged for determining such target SIR. The target SIR may be influenced by several factors, such as the travel speed of the UE and multipath signal propagation. The OLPC system may be relatively slow, with a frequency of update of for example 10 to 100 Hz for WCDMA (Wideband Code Division Multiple Access).
If the received signal at the BS from the UE has a SIR, which is above said target SIR, this is an indication that the UE is transmitting with unnecessarily high power. Then, the base station may send a signal to the UE to decrease the power. This may be performed by an inner loop power control also called a Fast Closed Loop Power Control FCL PC. This Power Control may operate with a frequency of update of for example 1.5 kHz for WCDMA. If the SIR is lower than the target SIR, the base station sends a signal requesting the UE to increase its power, and vice versa. Of course, the power can only be increased up to a certain upper limit.
In the same way, the reception of the signal transmitted by the base station is analyzed at the UE and the SIR of this signal is compared with a target SIR, and a signal is sent by the UE to the base station to increase the power, if the SIR is below the target SIR and vice versa.
Each base station may transmit a pilot signal on a Common PIlot CHannel CPICH. The signal strength of this pilot signal at the UE determines if the base station qualifies for being included in a list of active base stations maintained by each UE. The active list may be updated when the signal strength of a base station exceeds or is below certain values as described in more detail below. The list of active base stations may be limited to a specific number of base stations, for example six base stations.
When a specific UE moves from a cell area towards an adjacent cell area, the signal strengths from the base stations vary. When for example the signal strength of a specific base station becomes too low, or the signal strengths of another base station exceeds the signal strength of the specific base station, a handover situation may arise. There may be a soft handover or a hard handover.
In a hard handover situation, the UE may communicate with a single BS at each moment in time. If the signal strength of that BS, as determined by the pilot signal, decreases below a certain level, and another BS is available with a higher signal strength, the UE or BS initiate a hard handover event, in which the communication from and to an UE is transferred from one BS to a new BS.
In a WCDMA system, soft handover is foreseen. In soft handover the UE communicates with several BS:s simultaneously, which are within the reach of the UE. All signals received by the BS:s are delivered to the Radio Network Controller RNC, which may select the best signal among the signals received at the BS:s or combine the signals received at the BS:s.
The UE maintains a list of active BS:s, selected according to certain criteria as mentioned in more detail below. The UE receives the same messages from all BS:s included in the active list and combines these signals. Thus, a better reception may be obtained compared to receiving signals from a single BS, especially if there are several base stations with approximately equal signal strengths.
The above-mentioned system is disclosed in for example a book with the title: “WCDMA FOR UMTS Radio Access For Third Generation Mobile Communications” edited by Harri Holma and Antti Toskala.
WO 03/077584 discloses a device for controlling the power in an asymmetric soft handover condition in a communication network. The communications between the mobile station and the base station take place concurrently via two interface channels downlink from each base station separately. In uplink direction, the code channel of the mobile station is received by both base stations, but the received data is then routed to the associated radio network controller RNC for combining. Then, the RNC selects the better communication between the two possible radio links, and this selection takes place periodically.
EP 1223769 discloses a method for controlling uplink transmission power in a handover region by a UE in communication with a Node B using an FCS scheme. The UE stores TPC commands received for a specific duration from a plurality of cells in an active set, if the UE enters in the handover region during communication with a current best cell. If a next best cell is selected from the plurality of the cells, the UE determines a transmission power offset by comparing TPC commands from the current best cell with TPC commands from the next best cell for the specific duration at a time point where the best cell is changed from the current best cell to the next best cell. The UE transmits initial transmission power for the next best cell at a transmission power level determined considering the transmission power offset.
In cellular systems like WCDMA all users share the same radio resource. In order to optimize the system capacity, power control is introduced, as mentioned above, such that all users use sufficient power in order to reach sufficiently good Quality of Service. On the physical layer this is measured as block errors and typically a user should use only so much power that e.g. 1% of the blocks are erroneously received, nothing more nothing less. How much power a mobile terminal (UE) may use is often defined through requirements in the specifications.
Another feature of WCDMA, as well as other cellular systems like GSM, is mobility, i.e. the possibility to do calls when moving around for instance in a car. In such scenarios, handover is important. Furthermore, the handover should be reliable, i.e. when a UE or base station signals a handover to another base station, the handover message as well as the handover procedure should work. If a handover fails, the call may be dropped.
In order to reduce the drop probability in handover scenarios, the handover messages are often sent in acknowledge mode. That means that if a handover message was erroneously received, for instance in the base station, that information is fed back to the UE that makes a retransmission. However, retransmissions means delays, typically in the order of a second. In high speed scenarios, e.g. in a car driving on a highway, when the signal strength is very weak and a new stronger cell is detected and a handover message is transmitted from the UE, delays due to retransmissions might imply that the UE goes out-of-sync, i.e. loses the (down-link) connection to the old base station, before the handover procedure is finalized. This may cause dropped calls.
Another risk, especially seen in WCDMA systems where only sufficient output power from the UE is allowed, is that the base station goes out-of-sync, i.e. loses the (up-link) connection to the UE. In such a scenario, the base station does not receive the handover message at all, and therefore does not ask for retransmission. Again a dropped call may be the result.
Hence, there are needs for methods and apparatuses that increase the probability of received and correctly detected handover messages and therefore make cellular system like WCDMA more robust.